A radio receiver generally does not know whether or when a frame is emitted by a transmitter. The decision regarding whether or when a valid useful signal is present is therefore the responsibility of the receiver controller.
In the so-called frame reception mode, although the controller of the receiver can assume that the transmitter will transmit a frame, the receiver does not know the exact time. The receiver must therefore stay in a search mode for a particular time and must wait for the start of the useful signal. In a so-called polling mode, the receiver is activated at regular intervals in order to check whether a useful signal (wanted signal) is received. If a useful signal is detected, the receiver remains active for a period in order to be able to completely receive a frame.
Irrespective of the reception mode used, a receiver is supposed to be able to successfully detect a useful signal and distinguish it from other signals (for example interference signals, noise, etc.). Incorrect detection (failure to recognize a useful signal) may result in the useful signal not being recognized and in the message contained therein not being received and being ignored as a result. Furthermore, incorrect detection of an interference signal as a useful signal (“false alarm”) may result in the receiver being activated even though a message is not transmitted. In addition to the sequence of bit values which is to be transmitted and represents the information, a frame contains further parts, for example parts used for signal acquisition. At least the bits are represented by a sequence of modulation symbols which are emitted and need to be detected at the receiver end. Depending on the type of modulation used, each modulation symbol has a particular information content. In the case of unspread PSK modulation, one (binary PSK) or more bits (m-fold PSK) are transmitted for each modulation symbol. In the case of DSSS (direct-sequence spread-spectrum) transmission, a plurality of modulation symbols represent one bit. A message may consist of a plurality of frames.
Many implementations of a digital receiver provide for one or more parameters relevant to the demodulation to be determined before the reception signal is demodulated. For example, a frequency error in the carrier signal (also the phase error in the case of coherent demodulation) and/or the modulation symbol timing (phase angle of the modulation symbols) is/are determined. These parameters are determined with such an accuracy that data can be received (that is to say the reception signal can be demodulated). Parameter control can be optionally used to adjust the current values of the relevant parameters (for example symbol timing) on the basis of the current reception signal, in which case the initially recognized parameter values can be used as starting values.
The parameter detection mentioned which is needed to successfully demodulate the reception signal is part of the acquisition method carried out in the receiver. This method usually comprises a plurality of successive steps which are based on one another, a result obtained in one step not being able to be directly verified or falsified in the next step. In the event of an incorrect change from one acquisition step to the next, a long dwell time may be necessary in this step in order to avoid rejecting—possibly correct—useful signal reception.